1. Field of the Invention
The present invention relates to an interface protocol for digital communications, and more particularly, to an interface protocol for an asymmetrical digital subscriber line (ADSL) subscriber bus interface (SBI).
2. Background
The telecommunications industry has developed schemes for transmitting telephony signals in digital formats, for example, in the form of time division multiplexed (TDM) signals for transmission over a physical layer interface, such as a subscriber bus interface (SBI). The SBI receives data signals divided into frames. Each SBI frame includes time slots which are assigned payload data representing digitized telephony signals for conventional telephone services.
An example of a conventional telephone service is a plain old telephone service (POTS), which uses a digital format that is known to a person skilled in the art. An example of an SBI frame format for upstream transmission used in Litespan line cards made by DSC corporation of Plano, Tex. is shown in FIG. 1. The SBI frame format of FIG. 1 includes 32 time slots numbered consecutively from 0 to 31. Each of the slots within the conventional SBI frame includes 16 bits. As shown in FIG. 1, for time slot #0, abbreviated as xe2x80x9cTS0xe2x80x9d, bits are interleaved with reserved bits denoted by xe2x80x9cRxe2x80x9d, which are not assigned to carry information in the upstream SBI protocol.
POTS digital signal 0 bytes, abbreviated as xe2x80x9cDS0xe2x80x9d, are assigned to time slots #1-3, 5-15, 17-23, and 25-31 within the SBI frame. Each of the DS0 signals has a format similar to that of the TS0 cell. Each DS0 byte includes 8 bits which are interleaved with eight reserved bits similar to the interleaving of the TS0 byte with the eight reserved bits within time slot #0 as shown in FIG. 1.
In the SBI frame as shown in FIG. 1, time slot #4 is assigned to carry a xe2x80x9cSIGxe2x80x9d byte which identifies a signaling channel. The SIG byte in time slot #4 is required in the conventional SBI protocol and is known to a person skilled in the art. Time slot #16 is assigned a xe2x80x9cSRQxe2x80x9d byte which identifies a SBI service request channel. The SRQ byte in time slot #16 is required in the SBI communication protocol and is known to a person skilled in the art. Time slot #24 includes an SBI data link channel which is abbreviated as xe2x80x9cDLxe2x80x9d in FIG. 1. The DL byte in time slot #24 which is assigned to carry the SBI data link channel is required in the SBI communication protocol and is known to a person skilled in the art.
FIG. 2 shows a diagram of a conventional POTS channel bank where POTS line cards 102a, 102b, . . . 102n are connected to the backplane conventional bank control unit (BCU) 110 through a backplane interface such as a conventional SBI 106. In the conventional POTS SBI channel bank, each of the line cards 102a, 102b, . . . 102n is capable of supporting up to 24 conventional POTS DS0 channels and 4 conventional POTS control channels. A DS0 channel is 64 kilobits per second (kbps) digital channel each having a transmission rate of 8,000 bytes per second, each byte including 8 bits. Each of the DS0 channels supports one conventional POTS interface or its equivalent. SBI 106, which is on the backplane of the BCU 110, allows the BCU to process upstream DS0 signals from one or more POTS line cards 102a, 102b, . . . 102n. A conventional SBI channel bank system is capable of carrying upstream and downstream traffic separately, with the upstream traffic defined as transmission in the direction from one of the POTS line cards 102a, 102b, . . . 102n to the BCU 110, and the downstream traffic defined as transmission in the direction from the BCU to the POTS line card. The direction of the upstream traffic is indicated by arrows 104a, 104b, . . . 104n leading from the POTS line cards 102a, 102b, . . . 102n to the BCU 110, respectively.
For the upstream traffic, each of the line cards 102a, 102b, . . . 102n has a serial backplane traced to the BCU 110 through the SBI 106, which is physically provided on the backplane of the BCU 110. The conventional SBI frame format allows each of the line cards 102a, 102b, . . . 102n to support 32 channels each capable of providing a transmission rate of 64 kbps. The 32 channels may include 24 conventional DS0 channels, 4 control channels, 2 data link channels, a framing channel and a reserved channel. The DS0 channels, the control channels, the data link channel, the framing channel and the reserved channel are multiplexed in the time domain by using a conventional time division multiplexing (TDM) technique, which is known to a person skilled in the art. A clock 108 is provided for the BCU 110 and operates at a clock rate of 4.096 MHZ. In the conventional POTS SBI channel bank system, every other bit time on the clock 108 is unused, thereby producing an affective data rate of 2.048 MHZ, which is one half of the clock rate of 4.096 MHZ, on the backplane trace of the BCU 110. The effective data rate of 2.048 MHZ is sufficient to support 32 time slots, with each time slot sufficient for supporting a conventional 64 kbps channel. As shown in FIG. 1, a conventional SBI frame which has a duration of 125 xcexcs is long enough to carry one 8-byte per SBI time slot for each of the 32 channels each having a data rate of 64 kbps.
Broadband traffic of data in an asynchronous transfer mode (ATM) format is carried by conventional asymmetrical digital subscriber lines (ADSL). The ATM data are transmitted in the form of ATM cells assigned to the respective time slots in the SBI frames, each frame having a duration of 125 xcexcs. The transmission of the ATM cells over the ADSL is called xe2x80x9cATM over ADSL.xe2x80x9d Examples of digital signals which are carried in the ATM format include digital video signals and computer data signals. On the other hand, telephony signals are usually carried over the conventional DS0 channels which are time division multiplexed and assigned to the SBI time slots according to the SBI frame format as shown in FIG. 1. It is desirable to be able to transmit the conventional POTS DS0 channels and the ATM data within the same SBI frame. It is also desirable to be able to convey both ATM and POTS traffic over existing conventional channel bank backplanes.
However, the conventional SBI frame format as shown in FIG. 1 and described above is not capable of supporting both the POTS DS0 channels and the broadband ATM traffic with a desirable data rate, when each bit in the DS0 cells is interleaved with a reserved bit. Therefore, there is a need for an upstream subscriber bus interface protocol which allows an SBI frame to carry both POTS and ATM traffic. Furthermore, there is a need for an upstream SBI data format which allows an SBI frame to be processed by either a conventional narrowband BCU or a broadband ADSL bank control unit (ABCU). There is yet a further need for an upstream SBI protocol which allows mixed connections between the line cards and the control unit, for example, a mixed connection between a conventional POTS line card and a broadband ABCU, or a mixed connection between an ADSL line card and a conventional narrowband BCU.
The present invention satisfies these needs. In accordance with the present invention, a method of formatting an SBI frame comprises the steps of:
assigning asynchronous transfer mode (ATM) payload bytes to some SBI time slots within the SBI frame; and
assigning plain old telephone service (POTS) digital signal bytes to some of the SBI time slots within the SBI frame, wherein the SBI frame is configured for transmission from a line card selected from the group including an asymmetrical digital subscriber line (ADSL) line card and a POTS line card to a control unit selected from the group including an ADSL bank control unit (ABCU) and a narrowband bank control unit (BCU).
In one embodiment, the SBI frame includes an overhead byte. The control unit may receive upstream POTS and ATM bytes from different ports of a line card. The overhead byte includes back pressure indicator (BPI) bits for signifying to the control unit whether to enable or to disable downstream transmission on one of the channels from the control unit to one of the ADSL ports on the line card when the line card is an ADSL line card and the control unit is an ABCU. Each of the BPI bits may be assigned either xe2x80x9c0xe2x80x9d for disabling the downstream transmission within the respective channel or xe2x80x9c1xe2x80x9d for enabling the downstream transmission. The overhead byte may further include port identifier bits for identifying the port number of each port on the line card. The overhead byte may further include cell type indicator bits for indicating the type of cells in the SBI frame.
In a further embodiment, the method of the present invention comprises the additional steps of assigning an ADSL identification tag (AIT) byte to the SBI frame for identifying whether the line card is an ADSL line card and transmitting the AIT byte from the line card to the control unit. Furthermore, the AIT byte and the overhead byte may be assigned to the same time slot, such as SBI time slot #0, which is the first time slot within the SBI frame.
In another embodiment, the SBI frame according to the present invention includes a plurality of ATM header bytes, each of which is bit interleaved with a respective one of the POTS digital signal bytes within a respective one of the SBI time slots.
Furthermore, ATM header bytes may be assigned to a first plurality of the time slots, each of which includes, in addition to a respective one of the ATM header bytes, one of the POTS digital signal bytes. Each ATM header byte may be bit interleaved with a respective one of the POTS digital signal bytes within each of the first plurality of the time slots before the SBI frame is transmitted from the line card to the control unit.
Furthermore, in the embodiment in which the BPI bits are provided in the SBI frame for the upstream transmission from an ADSL line card to the control unit, and the BPI bits may be transmitted twice within each SBI frame to signify to the control unit whether to enable or to disable downstream transmission from the control unit to the line card. The BPI bits may be transmitted as part of the overhead byte within the first time slot, that is, time slot #0, and transmitted again within a time slot in the middle of the SBI frame, such as time slot #16.
Thus, with the present invention, both the conventional narrowband digital signal channels of a conventional POTS system and the wide band ATM payload data may be included within a single SBI frame for upstream transmission from a line card to a control unit. The present invention allows the reserved bits within the SBI time slots of a conventional SBI channel bank system to be utilized efficiently, thereby producing an effective data transmission rate twice that of the conventional SBI channel bank system without increasing the clock rate.
Further, with the present invention, compatibility between conventional narrowband line cards and a broadband ADSL bank control unit (ABCU), and between broadband ADSL line cards and the conventional narrowband bank control unit (BCU) can be obtained. Further with the present invention, the control unit which may be either an ABCU or a BCU, and can be connected to a mixture of POTS line cards and ADSL line cards, thereby solving the problem of compatibility between the SBI formats of ATM over ADSL and conventional POTS channels.